I agree about the 21.4MHz I.F. in a single conversion scheme. If the
required gain is achievable, this is a very good way to go. I remember one
of the Rockwell-Collins ADF receivers that had this setup.
As far as a 45MHz 1st I.F.............. these crystal filters seem to be
designed with a 455kHz 2nd I.F. in mind because a 910kHz rejection spec is
given. I have used that scheme over the years with a diode ring mixer as the
1st mixer. I follow the mixer with a series L, shunt C as the second leg of
a diplexer that matches the 50 ohm output of the mixer to the crystal
filter............ The first leg being is a parallel LC tuned circuit that
is in series with a 50 ohm idler resistor. With a reactance of the LC
elements of around 47 ohms, the selectivity at 6 db is approximately 15kHz.
The (2 time 2nd I.F. image rejection ) is about 60dB when using four poles
at 45MHz. I haven't done any in-band spur calculations with this scheme for
the aircraft band but I think that with high-side injection it would be ok.
Of course, if a 25kHz tuning step is used I am not sure how it would affect
the phase noise of the synthesizer if a Fractional N topology wasn't used. I
an NE602 is used as the 2nd mixer and just the I.F./ demodulator sections of
a TDA1572 were used, the only adjustment required would be a trimmer
adjustment of the 2nd LO in order to have a symmetrical adjacent channel
response.
Pete
wrote in message
oups.com...
From: "Pete KE9OA" on Fri, Mar 18 2005 6:36 am
I would go directly from 45MHz to 455kHz..............this is typical
in HF
receivers. An NE602 mixer can be used for the 2nd mixer and a
44.545MHz
crystal used with the on-board oscillator allows you to derive your
2nd I.F.
Alternatively, you can use a TDA1572 as the 2nd mixer/I.F./
demodulator.
This is also a good system and it will give you fairly good strong
signal
handling capability.
Pete
"Netgeek" wrote in message
...
Parts are on the way...
First IF = 45 MHz
Second IF = 10.7 MHz
Any point in going further to 455 KHz for a third IF or just stick
with the 10.7 - (MC1350 plus IF transformer) scheme?
Bill
As a suggestion (too late if parts are incoming), a
single conversion to a 21.4 MHz IF is quite suitable.
Using a monolithic quartz crystal filter (available
from DigiKey for about $15 a pair of two) between
the mixer and first IF could eliminate ALL tuned
circuits in the IF following the 1st IF amplifier.
Those are available in 12.5 KHz bandwidth which will
be fine for a VOR signal. [DigiKey has a link to
download specs for the ECS-made monolithics]
This is now common in mobile radios, both new and
retrofit of older ones.
If a single-conversion scheme with 21.4 MHz is done,
the LO can be 86.6 to 96.6 MHz with an image at
65.2 to 75.2 MHz. That is a low enough frequency
to allow a simple L-C "top coupled resonator" fixed
bandpass filter for the front end at 108 to 118 MHz
(8.9 % bandwidth).
Doing double conversion with a first IF of 45 MHz
is, by itself, no problem. However the 2nd IF image
is a bit too close to the nominal bandwidth of any
45 MHz 1st IF tuning (it's only 0.91 MHz away). With
the second's image (on either side depending on 2nd
LO above or below 45 MHz), there's still a chance to
pick up part of the FM BC band locally. To avoid
that, the 2nd LO should be on the high side of 45.
Second IF image would then fall into the low end of
the 118 to 137 MHz comm band (also AM) and those
transmitters are much lower powered ones than BC
stations.
With a 10.7 MHz 2nd IF, its image would be 21.4 MHz
away and rather easy to attenuate in the 45 MHz 1st
IF. There's only a slight problem using stock
10.7 MHz IF components: Bandwidth of the whole 2nd
IF might be around 160 KHz; less discrimination to
nearby VORs and Localizers. A study of FAA sectional
charts might be called for to check on potential
interfering stations although those are assigned in
regards geographic locations to minimize normal
interference.